Other theories include the "freeze thaw cycle", which supposedly can change the orientation of coins, burrowing rodents and other creatures which can push them around, and so forth. Not sure how much stock to put in these theories, but they are out there.
Minelab claims in Bruce Candy's paper on how metal detectors work that lightning well out of earshot can cause EMI to the machine. Less lightning, better results. (I believe there is a link to that paper around here somewhere, or at least there used to be; worth reading).
I had a friend who claimed the relative humidity affected his results. Don't know how much science he did on that theory. Never noticed a correlation myself.
I used to have a Detech 13 coil that was white. They said it was to reduce heat in the coil, so the engineer must have thought that there was something to the thermal theory. When that coil failed, the only replacement I could get was black. The engineers must have changed their mind. I never noticed a difference myself, but try to find a white coil and see if it makes a difference.